Add cycle in a linked list

Author: rp-code9

README.md

@@ -71,7 +71,8 @@ Algorithms [Part 1](https://www.coursera.org/learn/algorithms-part1/) and [Part2
 | Reverse a linked list | [link](https://leetcode.com/explore/interview/card/top-interview-questions-easy/93/linked-list/560/) | ? | [Iterative](sites/leetcode/ReverseList.cpp) <br/> [Recursive](sites/leetcode/ReverseList.cpp)|
 | Merge two sorted | [link](https://leetcode.com/explore/interview/card/top-interview-questions-easy/93/linked-list/771/) | ? | [Simple](sites/leetcode/MergeSortedList.cpp) |
 | Palindrome List | [link](https://leetcode.com/explore/interview/card/top-interview-questions-easy/93/linked-list/772/) | ? | [Double Reverse](sites/leetcode/PalindromeList.cpp) |
-| | [link]()| ? | []() |
+| Cycle in Linked List | [link](https://leetcode.com/explore/interview/card/top-interview-questions-easy/93/linked-list/773/) | ? | [Slow and fast pointer](sites/leetcode/LinkedListCycle.cpp) |
+| | [link]() | ? | []() |
 
 ## [Google CodeJam](sites/googlecodejam/README.md) 
 1. Google Code Jam 2018

sites/leetcode/LinkedListCycle.cpp

@@ -0,0 +1,116 @@
+// https://leetcode.com/explore/interview/card/top-interview-questions-easy/93/linked-list/773/
+
+#include <iostream>
+#include <vector>
+
+class Node
+{
+public:
+    int key;
+    Node* next;
+
+    Node(int x) : key {x}, next{nullptr} {}
+};
+
+class LinkedList
+{
+private:
+    Node* head;
+
+public:
+    LinkedList() : head{nullptr} {}
+
+    void printList() {
+        if (head == nullptr) {
+            return;
+        }
+
+        Node* temp = head;
+        while (temp != nullptr) {
+            std::cout << temp->key << " ";
+            temp = temp->next;
+        }
+        std::cout << "\n";
+    }
+
+    void printList(Node* node) {
+        if (node == nullptr) {
+            return;
+        }
+
+        while (node != nullptr) {
+            std::cout << node->key << " ";
+            node = node->next;
+        }
+        std::cout << "\n";
+    }
+
+    void addFront(int key) {
+        Node* node = new Node(key);
+
+        if (head != nullptr) {
+            node->next = head;
+        }
+
+        head = node;
+    }
+
+    void addKeys(const std::vector<int> keys) {
+        for (auto key : keys) {
+            addFront(key);
+        }
+    }
+
+    void createCycle() {
+        if (head == nullptr || head->next == nullptr) {
+            return;
+        }
+
+        Node* second = head->next;
+        Node* previous = nullptr;
+        Node* current = head;
+
+        while (current != nullptr) {
+            previous = current;
+            current = current->next;
+        }
+
+        previous->next = second;
+    }
+
+    bool hasCycle() {
+        if (head == nullptr) {
+            return false;
+        }
+        
+        Node* slow = head;
+        Node* fast = head->next;
+
+        while (fast != nullptr && fast->next != nullptr) {
+            if (slow == fast) {
+                return true;
+            }
+            
+            fast = fast->next->next;
+            slow = slow->next;
+        }
+
+        return false;
+    }
+};
+
+int main()
+{
+    std::vector<int> keys {5, 1, 0, -3, -99};
+    // std::vector<int> keys {5, 1};
+    
+    LinkedList list;
+
+    std::cout << "List : ";
+    list.addKeys(keys);
+    list.printList();
+
+    list.createCycle();
+
+    std::cout << "List has cycle : " << std::boolalpha << list.hasCycle() << "\n";
+}